1. Field of the Invention
The present invention relates to a suspension apparatus of an automotive vehicle and, more particularly, to a suspension apparatus of an automotive vehicle so arranged as to alter its suspension characteristics in a desired manner.
2. Description of Related Art
Heretofore, a suspension apparatus as called a passive suspension comprises a damper unit consisting of a hydraulic damper and a spring such as a coil spring. This passive suspension apparatus is arranged such that the suspension characteristics are set substantially uniform although the damping force of the hydraulic damper is variable yet its range is so small.
On the other hand, there has been proposed a suspension apparatus as called an active suspension having a liquid cylinder unit interposed between a sprung weight and an unsprung weight, which is so arranged as to alter its suspension characteristics in a desired manner by controlling the amount of an operating fluid to be supplied to or discharged from the fluid cylinder unit, as disclosed in, for example, Japanese Patent Publication (kokoku) No. 14,365/1984 and Japanese Patent Laid-open Publication (kokai) No. 130,418/1988.
Generally, the vibration of an automotive vehicle may be divided into three kinds, i.e. bounce, pitch and roll. Such an active suspension has a fluid cylinder unit disposed at each wheel. In order to improve comfort and running stability against such three kinds of vibration, the active suspension is so arranged as to control an opening angle of a flow rate control valve for each wheel by a given control gain so set as to control the quantity of the operating fluid to be supplied to or discharged from the fluid cylinder unit for each wheel in accordance with the running state of the automotive vehicle. Hence, the active suspension is provided with vehicle height controlling means for controlling the vehicle height of the automotive vehicle to reach a target vehicle height and with at least one of vehicle-height displacing speed controlling means for controlling a speed of displacing the vehicle height and acceleration controlling means for reducing vibration of the automotive vehicle body, thereby controlling the opening angle of the flow rate control valve for each wheel by means of each controlling means by operating the quantity of the operating fluid to be supplied to or discharged from the fluid cylinder unit for each wheel in accordance with the desired purposes, adding the values and calculating the quantity of the operating fluid to be supplied to or discharged from the fluid cylinder unit for each wheel.
For the active suspension apparatus, the quantity of the operating fluid to be supplied to or discharged from the fluid cylinder unit is increased when a high-frequency vibration is applied to the automotive vehicle. As a result, there may be the risk of causing an oscillating phenomenon resulting as a matter of course from the control of the active suspension being performed by feedback control and from a delay in response due to a detecting system, a control system, a mechanic system, etc. Hence, it is general that the level of a control signal to an actuator for driving the flow rate control valve for each wheel is so controlled as to be lowered in a region in which the high-frequency vibration is occurring, by inputting an operating signal indicative of the quantity of the operating fluid operated by each controlling means into a flow rate filter having a filter constant as represented by 1/(1+.tau.S) (where .tau. is a time constant and S is a Laplace's operator). In other words, as the Laplace's operator gets larger as a frequency increases, the level of the control signal to be generated to the actuator from the flow rate filter is decreased in the region in which a high-frequency vibration is occurring by inputting the operating signal indicative of the quantity of the operating fluid to be supplied to or discharged from the fluid cylinder unit into the flow rate filter having such a filter constant, thereby preventing the oscillating phenomenon from occurring.
In the active suspension using the flow rate filter having the filter constant as represented by 1/(1+.tau.S), attempts have been made to prevent the oscillating phenomenon from occurring in the region in which high-frequency vibration occurs, exclusively by means of the Laplace's operator S that increases as the frequency increases because the time constant .tau. is a constant. As the oscillating phenomenon is caused to occur when the level of the control signal is caused to be elevated and a speed at which the operating fluid is supplied to or discharged from the fluid cylinder unit becomes great, some active suspensions cannot reduce the level of the control signal to be outputted to the actuator to a sufficiently low level by the flow rate filter when the level of the operating signal indicative of the operated quantity of the operating fluid to be supplied to or discharged from the fluid cylinder unit and the speed of the operating fluid supplied to or discharged from the fluid cylinder unit is high. Hence, they present the problem that the oscillating phenomenon cannot be prevented in a desired way from occurring in the region where the high-frequency vibration occurs.